1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine, a printer, a facsimile apparatus, or a multifunction peripheral. Further, the present invention relates to a configuration of an image forming apparatus including an optical sensor for optically detecting a toner image formed on an image bearing member.
2. Description of the Related Art
A conventional electrophotographic image forming apparatus forms toner images on photosensitive drums (image bearing members), transfers each toner image on an intermediate transfer member (a belt member or an image bearing member) to overlay the toner images, transfers the toner images onto a recording material conveyed, and fixes the toner images on the recording material. Such an image forming apparatus includes an optical sensor that detects misregistration of toner images (hereinafter referred to as toner patches). The optical sensor is typically located opposite a surface of the intermediate transfer member.
The optical sensor typically includes a light-emitting element, a light-receiving element, electrical circuit boards for driving each element, a lens, and a sensor frame.
Examples of optical sensors include a specular reflection type sensor 300 as illustrated in FIG. 10 and a diffuse reflection type sensor 400 as illustrated in FIG. 11. The specular reflection type sensor 300 detects specular reflected light. The diffuse reflection type sensor 400 detects diffused light. In the specular reflection type sensor 300, light emitted from a light-emitting element 31 is condensed via a lens 32. The condensed light is reflected by the surface of an intermediate transfer member 35. The reflected light is condensed onto a light-receiving element 34 via a lens 33. The light-receiving element 34 receives the condensed light and produces a detection signal (e.g., voltage signal) indicative of intensities of the condensed light received therein. The specular reflection type sensor 300 detects a misregistration of a plurality of toner patches based on a change of detection voltages generated in the light-receiving element 34. The specular reflection type sensor 300 is used in a case where the amount of light incident on the surface of the intermediate transfer member 35 is relatively large and the amount of light reflected by the intermediate transfer member 35 is sufficiently large compared with the amount of light reflected on the toner patches to be read (see FIG. 12).
On the other hand, the diffuse reflection type sensor 400 works in a manner similar to the above-described specular reflection type sensor 300, but it is selected in a case where the amount of light incident on the surface of the intermediate transfer member is relatively small and the amount of light reflected on the toner patches to be read is larger than the amount of light reflected from the surface of the intermediate transfer member (see FIG. 13). However, in the diffuse reflection type sensor 400, since output signals of the color toner patches of magenta (M), yellow (Y), and cyan (C) are opposite in signal level to an output signal of the black toner patch (Bk), the center of gravity of a color region cannot be calculated based on a constant output signal. Therefore, it is necessary to superpose other color toner patches on the black toner patch to express edge portions of the black toner patch.
In any one of the above-described optical sensors, the following method may be adopted for attaching the sensor inside an sensor frame. As discussed in Japanese Patent Application Laid-Open No. 2004-309292, generally, a light-emitting element and a light-receiving element of the lead type, or an electrical circuit board including a light-emitting element and a light-receiving element (a light-emitting unit and a light-receiving unit), are fixed at a predetermined position inside a sensor frame via a solder so as to cause an angle of emission of the light-emitting element and an angle of incidence of the light-receiving element to be within a predetermined range.
In the case where the light-emitting unit attached to the sensor frame is used, an technician (operator) determines a position of the electrical circuit board relative to the sensor frame while adjusting an inclination of an optical axis of the light-emitting element, and then fixes the electrical circuit board to the sensor frame using a fixing tool such as a screw. However, in such an operation, the fixing tool presses the electric circuit board and generates a force that distorts the shape of the electrical circuit board, which in turn changes the parallelism of the electrical circuit board. As a result, the light-emitting optical axis (dark arrow in FIG. 14) inclines with respect to a fixing plane as illustrated in FIG. 14, so that an illuminance distribution becomes irregular as illustrated in FIG. 14, thus decreasing reading accuracy.
Since the inclination of the optical axis of the light-emitting element greatly affects the reading accuracy, it is desirable that the amount of the inclination be limited to a minimum or entirely avoided. More specifically, when the optical axis inclines, a signal of the toner patch read by the light-receiving unit is distorted, for example, as illustrated in FIG. 15B. As a result, in a case where the position of the center of gravity of the read signal is set to be a position of the toner patch, the sensor may detect as if each color exhibits a relative misregistration (hereinafter referred to as a color misregistration). For example, FIG. 15B illustrates a case where the position of center of gravity for each color is shifted by an amount that would result in color misregistration. If the misregistration is corrected based on a different value from the actual color misregistration, the color misregistration may be increased.
Further, in the case of a sensor for detecting the density of a toner image, other than the sensor for detecting the relative misregistration, the inclination of the optical axis may cause an obstruction in improvement of the accuracy of density detection.
Therefore, when an image forming apparatus uses a sensor for optically detecting a toner image, it is highly desirable to minimize or eliminate the inclination of the optical axis of a light-emitting element attached in the sensor.